1. Field of the Invention
The present invention relates to a scanning and exposing device for exposing a photosensitive drum to a laser beam output from a semiconductor laser oscillator while scanning the photosensitive drum with the laser beam, and an image forming apparatus such as a digital copying machine or the like for forming an electrostatic latent image on the photosensitive drum by the exposure of the scanning and exposing device and coating developing agent on the electrostatic latent image to form an image.
2. Description of the Related Art
With respect to an image forming device such as a digital copying machine using a semiconductor laser (hereinafter referred to as laser) as a light source, there is generally adopted a construction that the light emission amount of a laser in an image area is detected by a built-in photodiode or a photodiode provided outside the laser, and the stabilizing control of the light emission amount (which is the control generally called as APC: Auto Power Control, hereinafter abbreviated as APC) is carried out by using output current from the photodiode.
The APC processing is the processing of setting the light amount of the laser on the laser light source to a predetermined value, however, some factor for varying the laser light amount exists on the optical path from the laser light source to the photosensitive drum in the actual image forming device. For example, the laser light amount is not uniform in the main scanning direction on the surface of the photosensitive drum due to the difference in transmittance of the optical element.
The transmittance of the optical element is varied in accordance with the incident light angle, and there is generally a characteristic that the transmittance is larger as the light is incident more vertically to the surface of the optical element and the transmittance is smaller as the light is more obliquely incident to the surface of the optical element. The incident angle of the laser beam in the main scanning direction to an f-θ lens used in an image forming device or the like is more vertical in the neighborhood of the center and more oblique as it approaches more closely to the end. Therefore, the transmittance of the optical element at the position in the main scanning direction on one line is larger at the upper side of the ordinate axis than that at the lower side of the ordinate axis. Accordingly, the difference in transmittance in the main scanning direction described above causes a problem that even when the laser light amount on the laser light source is made constant by APC, with respect to the laser light amount in the main scanning direction on the surface of the photosensitive drum, the laser light amount is larger at the center portion of the photosensitive drum where the transmittance of the optical element is larger and also the laser amount is smaller as it approaches more closely to the end of the photosensitive drum where the transmittance of the optical element is smaller.
Furthermore, the transmittance of the optical element has a characteristic varying in accordance with the difference in maker and type.
With respect to factors for varying the laser light amount in the laser optical path such as the difference in transmittance of the optical element as described above, etc., there has been considered a method in which a correction value for the laser light amount in connection with each position in the main scanning direction is set in a correction value storage portion such as a memory or the like in advance, and the laser light amount is varied in accordance with the position in the main scanning direction under scanning, so that the laser light amount on the photosensitive drum in the main scanning direction is fixed.
Still furthermore, a method of reducing the number of correction values for correction of light amount in the main scanning direction is described in JP-A-2000-292934.
However, the above-described method of reducing the number of correction values for correction of light amount is applied to the case where one light source is used. For example, in the case where four light sources are provided for black (K), cyan (c), magenta (M) and yellow (Y) (hereinafter, black, cyan, magenta and yellow are fitly represented by K, C, M and Y) in a quadruple tandem transfer color machine, it is required to provide light amount correction values in the main scanning direction for each of the four light sources and thus the number of set values of the correction amount is increased. When the number of set values for the correction amount is increased, there is a problem that the cost is increased.